The ASPEN Model

The computer simulation model used to estimate toxic air pollutant concentrations is called the Assessment System for Population Exposure Nationwide, or ASPEN. This model is based on the EPA’s Industrial Source Complex Long Term model (ISCLT) which simulates the behavior of the pollutants after they are emitted into the atmosphere. ASPEN uses estimates of toxic air pollutant emissions and meteorological data from National Weather Service Stations to estimate air toxics concentrations nationwide.

The ASPEN model takes into account important determinants of pollutant concentrations, such as:

rate of release

location of release

the height from which the pollutants are released

wind speeds and directions from the meteorological stations nearest to the release

breakdown of the pollutants in the atmosphere after being
released (i.e., reactive decay)

settling of pollutants out of the atmosphere (i.e., deposition)

transformation of one pollutant into another (i.e., secondary formation)

The model estimates toxic air pollutant concentrations for every census tract in the continental United States, Puerto Rico and the Virgin Islands. Census tracts are land areas defined by the U.S. Bureau of the Census and typically contain about 4,000 residents each. Census tracts are usually smaller than 2 square miles in size in cities, but much larger in rural areas.

For some pollutants, the concentration estimates include a "background" concentration which is based on monitored values. Background concentrations are the contributions to outdoor air toxics concentrations resulting from natural sources, persistence in the environment of past years' emissions, and long-range transport from sources that are more than 50 kilometers away. In other words, background concentrations are levels of pollutants that could be found in 1996 even if there had been no manmade emissions in the U.S. (For more information, see Background Concentrations).